1. Field of the Invention
The present invention relates to a frame structure for a communication system, or more particularly, to a frame structure that withstands electromagnetic interference (EMI) or electrostatic discharge (ESD).
2. Description of the Related Art
A communication system is conventionally mounted or stored in a device frame such as a metallic housing shaped like a box and made by mounting a metallic cover on a framework, and wherein a power unit, a transmitter, a receiver, and other multiple electronic units are placed on shelves.
The whole communication system is covered with metal, and therefore, electromagnetic waves radiating from various units in the device frame will not leak out and the external electromagnetic waves will not enter the device frame, thereby minimizing the occurrence of a fault.
When a person touches a device frame such as a metallic housing, static electricity develops to induce a spark discharge. Then, the discharge voltage rapidly falls to 0 volts. As a result, a high-speed surface current (heavy current composed of short pulses having sharp leading edges) flows in the metallic housing, generates a localized magnetic field on the orthogonal axis, and then dies out.
Therefore, the magnetic field develops a wide-band electromagnetic wave with frequencies ranging from several tens of MHz to several GHz. Various electronic units in the device frame are affected. More particularly, (1) with the inflow of a heavy current that is composed of short pulses having sharp leading edges, the wiring of digital equipment is subject to induction noise (induction interference). A heavy current resulting from a discharge generates an electromagnetic field, which involves the wiring (electromagnetic induction), resulting in a noise voltage, and fluctuation of the signal level, thereby causing a malfunction. (2) The potentials of a frame ground (FG) line and a signal ground (SG) line are disturbed (fluctuated). The voltage of the SG line that is designed to be 0 volts differs greatly from circuit to circuit, thereby leading directly to a logical inversion and resulting in a malfunction.
In an effort to prevent a malfunction due to an electrostatic discharge through the human body, another large metallic frame (referred to as an accommodation frame) has been prepared differently from the device frame and the device frame has been stored in the accommodation frame in the past. This double structure allows an electrostatic discharge to occur at a place separated by more than a given distance from the device frame and thus prevents an electromagnetic wave, developed by the electrostatic discharge, from invading the device frame. The accommodation frame and device frame are grounded via independent earthing wires, so that a heavy current developed by an electrostatic discharge will not flow in the device frame.
The influence of an electrostatic discharge is inversely proportional to the square of the distance from the electrostatic discharge. A larger accommodation frame results in a larger distance. Consequently, the influence of an electrostatic discharge is reduced. However, the whole apparatus becomes larger, and thus, the operability of the apparatus deteriorates, and the price of the apparatus increases. Thus, the product value of the apparatus is degraded. So far as a communication system is concerned, diverse layouts are conceivable; that is, a transverse layout in which multiple units are arranged in a row, and a longitudinal layout in which multiple units are arranged back to back. To cope with the diverse layouts, different accommodation frames must be constructed in association with the layouts poor flexibility ensues.